The present application relates to a semiconductor device using a semiconductor molecule as a channel material and to method for manufacturing the same. More specifically, the present application relates to a semiconductor device which has a channel region with excellent electric conductivity and which is easy for the preparation and to a method for manufacturing the same.
Organic semiconductor devices of the related art, for example, organic transistors have a structure represented by of an electrode/molecular aggregate/electrode figure, dispose an organic semiconductor molecule material in a form of a molecular crystal, a coagulated molecular aggregate or the like between the two electrodes and form a channel layer between the electrodes. In such case, plural molecules intervene in a passage of a current flowing from the one-sided electrode to the other electrode, and the electric conduction in the channel layer includes an intramolecular charge transfer process wherein a charge transfers within a molecule and an intermolecular charge transfer process wherein a charge transfers between molecules from one molecule to a next molecule. In general, the mobility between molecules is much smaller than that within a molecule. As a result, the mobility of the channel layer configured by the molecular aggregate is restricted by the intermolecular charge transfer process and is small.
On the other hand, in the nanotechnology which is a technique for observing, preparing and utilizing a fine structure with a size of about 10 nm (=10−8 m), studies for making an individual molecule exhibit a function as an electronic part such as single molecular transistors attract attention.
Such a molecular device has a structure represented by of an electrode/molecule/electrode figure, and the semiconductor molecule interposed between the two electrodes is connected to each of the two electrodes at both ends thereof. Since the electric conduction between electrodes in such a molecular device including a monomolecular device is dominated by the intramolecular charge transfer process but not subjected to rate-determining by the intermolecular charge transfer process, rapid electric conduction becomes possible.
FIG. 14A is a diagrammatic view showing a structure of a molecular transistor 100 as described in J. Park, A. N. Pasupathy, J. I. Goldsmith, C. Chang, Y. Yaish, J. R. Petta, M. Rinkoski, J. P. Sethna, H. D. Abruna, P. L. McEuen, D. C. Ralph, Nature, 417, 722 (2002) (Non-Patent Document 1); and FIG. 14B is a structural formula showing a structure of a semiconductor molecule. As illustrated in FIG. 14A, this molecular transistor 100 is configured as an insulating gate type field effect transistor of a bottom-gate type. A substrate 101 also acts a gate electrode and is, for example, a silicon substrate having n-type conductivity by doping with n-type impurities. A gate insulating film 102 composed of silicon oxide is formed on the surface of the substrate 101 by means of thermal oxidation of the substrate 101; and a source electrode 103 and a drain electrode 104 each of which is composed of gold are provided opposite each other thereon. A semiconductor molecule 105 is disposed so as connect the source electrode 103 and the drain electrode 104 each other on the gate insulating film 102.
As illustrated in FIG. 14B, the semiconductor molecule 105 is a cobalt(III) complex containing two terpyridyl groups as a ligand and has a thiol group (—SH) which is easily adsorbed on gold or the like via an alkylene chain (—(CH2)n—) at both ends thereof The semiconductor molecule 105 is bound to the source electrode 103 and the drain electrode 104, respectively by this thiol group. Though a distance between the two thiol groups in the semiconductor molecule 105 varies a little with the length of the alkylene chain (—(CH2)n—), it is from about 2 to 3 nm.
In the molecular transistor 100, a current flowing between the source electrode 103 and the drain electrode 104 by means of intramolecular conduction of the semiconductor molecule 105 is modulated by a gate voltage to be applied to the gate electrode and functions as an insulating gate type field effect transistor. As easily surmised from FIG. 14A, in order that the semiconductor molecule 105 may form good junction to the electrodes 103 and 104, respectively by the thiol group present at the both ends thereof and act as a molecular device with high reliability, it is important that opposing electrodes having a gap part of substantially the same size as the length of the semiconductor molecule 105 are formed with good reproducibility.
Non-Patent Document 1 describes that the molecular transistor 100 was prepared by integrally forming the source electrode 103 and the drain electrode 104 connected each other via a thin stripe-like connecting part by means of patterning by electron beam lithography, adsorbing the semiconductor molecule 105 on this surface and then cutting the connecting part by means of electromigration, thereby separating it into the source electrode 103 and the drain electrode 104, respectively.
Also, S. Kubatkin, A. Danilov, M. Hjort, J. Cornil, J. Bredas, N. Stuhr-Hansen, P. Hedegard, T. Bjornholm, Nature, 425, 698 (2003) (Non-Patent Document 2) describes an example in which a monomolecular transistor having the same structure as in the molecular transistor 100 is prepared by using a phenylene vinylene oligomer having a thiol group at both ends thereof as a semiconductor molecule. In that case, an example in which opposing electrodes having a gap part of from 2 to 3 nm are formed by means of “oblique vapor deposition” using a shadow mask is reported. In all of Non-Patent Documents 1 and 2, the transistor operation utilizes coulomb brocade, and its measurement is carried out at a very low temperature.
As described in the foregoing examples, in many cases, the natural length of the semiconductor molecule to be used in the molecular device is several nm at longest. Therefore, the molecular device having a structure represented by an electrode/molecule/electrode figure and utilizing intramolecular conduction has no choice but to have a structure in which a semiconductor molecule having a length of several nm is disposed between opposing electrodes having a gap part of several nm and connected to the respective electrodes at the both ends thereof.
As to a method for manufacturing such a molecular device, various methods have been proposed until now. Such a manufacturing method can be roughly classified into a method in which opposing electrodes having a gap part of several nm are first prepared, and a semiconductor molecule is then disposed in the gap part (method 1) (see C. Kergueris, K. P. Bourgoin, S. Palacin, D. Esteve, C. Urbina, M. Magoga, and C. Joachim, Phys. Rev. B, 59, 12505-12513 (1999) (Non-Patent Document 3)); and a method in which one of electrodes is first prepared, a semiconductor molecule is then disposed thereon, and thereafter, the other electrode is prepared (method 2) (see Adi Salomon, David Cahen, Stuart Lindsay, John Tomfohr, Vincent B. Engelkes, and C. Daniel Frisbie, Adv. Mater., 15, 1881-1890 (2003) (Non-Patent Document 4)).
Also, (T. Sato, H. Ahmed, D. Brown, B. F. G. Johnson, J. Appl. Phys., 82, 696 (1997) (Non-Patent Document 5) reports a field effect transistor connecting a source electrode and a drain electrode each other by a gold fine particle chain connected by 1,6-hexanedithiol. The structure of this transistor is slightly different from that of the foregoing molecular device represented by an electrode/molecule/electrode figure, the charge transfer between molecules is not included in a conductive route, and the mobility is not restricted by the charge transfer between molecules; and therefore, rapid electric conduction becomes possible. Also, there is an advantage that the transistor can be prepared by employing electron beam lithography in a length of a gap part between the source electrode and the drain electrode is 30 nm.
According to the method 1 as described in Non-Patent Document 3, since the semiconductor molecule is introduced in the last stage of the manufacturing process, the semiconductor molecule is not damaged. On the other hand, it is not easy to prepare two electrodes having a gap part of several nm in length with high precision and with high reproducibility, and in many cases, it is difficult to form junction of the semiconductor molecule to the electrodes as expected. For that reason, the preparation of a molecular device having high reliability with good reproducibility has not been achieved yet.
On the other hand, according to the method 2 as described in Non-Patent Document 4, since it is not necessary to previously prepare opposing electrodes having a gap part in conformity with the length of the semiconductor molecule, this method is advantageous in view of the formation of junction of the semiconductor molecule to the electrodes. But, there is a problem that in forming a second electrode on a semiconductor molecular film, the semiconductor molecular film is damaged, or an electrode material comes into a defect present in the semiconductor molecular film, thereby unexpectedly affecting characteristics of the device (see Amy V. Walker, Timothy B. Tighe, Orlando M. Cabarcos, Michael D. Reinard, Brendan C. Haynie, Sundararajan Uppili, Nicholas Winograd, and David L. Allara, J. Am. Chem. Soc., 126, 3954-3963 (2004) (Non-Patent Document 6)). Accordingly, a method for forming a second electrode in which the semiconductor molecular film is less damaged is considered necessary.
Also, according to the method 2, in many cases, the molecular device is prepared by stacking up configuration components in a vertical direction so as to form a semiconductor molecular film on a first electrode and further forming a second electrode thereon. In such a device of a vertical type, a gate electrode must be disposed in a side of the semiconductor molecular film, and it is difficult to form a gate insulating film or control its thickness as compared by general devices of a horizontal type.
Also, in the field effect transistor as reported in Non-Patent Document 5, several gold fine particles having a particle size of about 10 nm are disposed between two electrodes having a gap part of about 30 nm, and connection between the electrode and the gold fine particle and connection between the gold fine particles are each achieved by 1,6-hexanedithiol. According to this method, it is possible to achieve connection between the electrodes by a repeating structure of molecule/gold fine particle/molecule/ . . . /gold fine particle/molecule without damaging the 1,6-hexanedithiol molecule. Moreover, this method is free from the problem in disposing a gate electrode as in the device of a vertical type. However, according to the method as reported in Non-Patent Document 5, since the disposition of the gold fine particle between the electrodes is left to the coincidence, it may be impossible to control how many and what places repeating structures are formed in the electrode. Also, there is a possibility that the gold fine particle adsorbed in the surroundings of the electrode unexpectedly forms a conductive route.